mingw: document the experimental standard handle redirection

This feature is still highly experimental and has not even been
contributed to the Git mailing list yet: the feature still needs to be
battle-tested more.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/git.txt

@@ -1186,6 +1186,23 @@ of clones and fetches.
 	  - any external helpers are named by their protocol (e.g., use
 	    `hg` to allow the `git-remote-hg` helper)
 
+`GIT_REDIRECT_STDIN`::
+`GIT_REDIRECT_STDOUT`::
+`GIT_REDIRECT_STDERR`::
+	(EXPERIMENTAL) Windows-only: allow redirecting the standard
+	input/output/error handles. This is particularly useful in
+	multi-threaded applications where the canonical way to pass
+	standard handles via `CreateProcess()` is not an option because
+	it would require the handles to be marked inheritable (and
+	consequently *every* spawned process would inherit them, possibly
+	blocking regular Git operations). The primary intended use case
+	is to use named pipes for communication.
++
+Two special values are supported: `off` will simply close the
+corresponding standard handle, and if `GIT_REDIRECT_STDERR` is
+`2>&1`, standard error will be redirected to the same handle as
+standard output.
+
 
 Discussion[[Discussion]]
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